FINDING an actor to play Dr Jekyll and Mr Hyde in a new modern-day version of the classic Robert Louis Stevenson story was easy for writer Steven Moffatt. "We made one of those lists," he recalls.

"You list every single actor you can think of with a huge BBC1 following, the acting chops to pull off a famously impossible part, enough charm to beguile you as Hyde even as he terrifies you, enough darkness to keep you on edge as Jekyll even as he breaks your heart... and then you look at that list and realise that if James Nesbitt says no, you're clean out of ideas."

It's a role - two roles, to be accurate - that's been played in past incarnations on screen by the likes of Spencer Tracy, John Barrymore, Jack Palance, David Hemmings, Anthony Perkins, Michael Caine and John Malkovich.

One big difference is that Moffatt's version doesn't portray the evil Hyde as a wolf-man. He's more attractive than Jekyll, or Jackman as he's known. "This is about a different man, not a different face," says Moffatt, who's written the comedy series Coupling and several Dr Who episodes.

"Ninety per cent of our Jekyll and Hyde distinction is about performance. Our Hyde acts different, rather than looks different. It's a story of subtly different men.

"Victorian evil is fanged and monstrous and different from us. Modern evil is seductive and attractive and looks pretty much exactly like us. This is a modern Dr Jekyll, using modern technology to contain his dark side. Mr Hyde is a modern take on evil - seductive and funny rather than twisted and monstrous."

Nesbitt, for his part, admits he was desperate to do the series after seeing the script. He read Stevenson's book a couple of times and watched a number films that have stayed with him, Jeff Bridges in Starman and Natural Born Killers, as part of his research.

"I also found that I could use pieces of myself that probably haven't been allowed to surface since I was a little boy and my mum told me to stop showing off, which has been incredibly liberating, very therapeutic."

He reckoned it was too simple just say that everyone has a dark side. "As much as we want to lead good, decent and happy lives, I think we're also attracted by something that may lurk within ourselves and the people around us. I play Jackman and Hyde as two very different characters who had to be totally different in the way they behave," he explains.

"One of the most important things is to make the fantastical believable. Hyde is very confident and grand-standing, larger than life, and that was a bit scary at first, a bit like diving off a board really.

"He's also very single-minded. He has no fear or moral conscience, no social or moral responsibility. He wants to eat, drink, have sex, dominate and just be. And he's a child. We meet him moments after his birth. He's a show-off, and being given an opportunity like that is probably what most actors crave. And to show off very good writing is great."

The physical transformation is very subtle, something that was decided upon after a lot of make-up tests. They wanted the audience to do a double take and ask themselves what exactly was the difference between the two men.

"The eyes are a different colour. It's amazing how simple an effect like black contact lenses can be, not just from a visual point of view, but also in terms of character, simple but shocking," says Nesbitt.

"There's a very slight elongation of the nose, a protrusion of the chin, which makes the face more angled, a subtle difference in the ears and a bit of a difference in weight - and it's great, I finally get to wear wigs. I've been saying to make-up artists for years that I want a full head of curly black hair and now I've got it.

"It's not that often that you get the chance to have a face you really want. In the past, I've said that it's very depressing to hand over the wigs at the end of the day, but now I also have to hand over my chin, nose and ears."

* Jekyll begins on BBC1 tonight at 9pm.